Ligand Fast Chemical Reactions

The electrochemical reduction proceeds at the carbene ligand as a two-electron irreversible process. Since a single reduction wave is observed, the first reduction step must be followed by a very fast chemical reaction yielding the intermediate (probably of radicalic nature), which is reduced even more easily than the starting compound to a final product (the so-called electron transfer, chemical reaction, electron transfer (ECE) mechanism). To understand the mechanism more deeply, the products of electrochemical reduction were separated during the preparative electrolysis from the solution using a continuous extraction to hexane, isolated, and analyzed. 

To obtain satisfactory results, several conditions must be fulfilled. The chemical reaction involved must be stoichiometric, no side reactions may occur or at least should be reproducible and quantitatively describable (such as the reaction of protons with ligands in chelatometric titrations) and the main reaction should have a high equilibrium constant value. The ISE should respond only to the titration system and the response should be fast and reproducible. 

In the second four-electron mechanism, electrochemical reaction of the Si-Si backbonds with holes is assumed to be fast in comparison with electrochemical ligand exchange or chemical reaction with HF or H2O. For the case shown in Fig. 32,... 

Electroanalyhcal techniques (also in combination with other techniques, e.g., ophcal techniques such as photometry and Raman spectrometry) can be employed to inveshgate many functional aspects of proteins and enzymes in particular. It is possible to study the biocatalytic process with respect to the chemistry of the active site, the interfacial and intramolecular ET, slow enzyme achva-tors or inhibitors, the pH dependence, the transport of tlie substrate, and even more parameters. For example, slow scan voltammetry can be used to determine the relation of ET rates or of protonation and ligand binding. In contrast, fast scan voltammetry allows the determination of rates of interfacial ET. In addition, it is also possible to investigate chemical reactions that are coupled to the ET process, such as protonation. The use of direct ET for mechanistic studies of redox enzymes was recently reviewed by Leger and Bertrand [27]. Mathemahcal models help to elucidate the impact of different variables on the enhre current signal [27, 75, 76]. 

For the MOF [Cu2(bpdc)2(dpq)2(H20)]-H20 obtained from the hydrothermal reaction of copper chloride with the mixed ligands bpdc and dpq, the redox couple observed at -100/-l-50mV versus SCE in pH 2 (phosphate buffer) was attributed to Cu /Cu . The charge below the reduction peak was much larger than that below the oxidation peak, possibly due to the instability of the reduced form of copper, which may undergo a fast chemical oxidation in an aqueous solution of pH 2. 

---